The present invention relates to a process and apparatus for the formation of high energy neutral atom beams by multiple neutralization. A preferred application thereof is in the heating of plasmas of thermonuclear reactors.
In order to fulfill the conditions necessary for producing energy by thermonuclear reactions with a fuel mixture (e.g. deuterium and tritium) one of the possible methods is the magnetic confinement of a plasma. In this case, it is necessary to obtain a particle density of 10.sup.20 m.sup.-3 and simultaneously a temperature of 10 keV or .about.10.sup.8 K. for a plasma confinement time of one second (Lawson criterion).
In existing toroidal plasma machines, the small torus radius is that of the plasma and is equal to approximately 2 meters, the large torus radius being equal to approximately 10 meters, whilst the toroidal magnetic confinement field is 3 to 4 Teslas.
The heating of such a plasma by an electron current (ohmic heating) is limited to approximately 1 to 2 keV due to the reduction in the resistivity of the plasma when the temperature thereof rises. For this reason, it is necessary to add an auxiliary heating to increase said temperature and pass it from 2 to 10 keV.
One of the known auxiliary heating means consists of injecting into the plasma fast particles produced outside the plasma and which enter the reactor through one or more openings made in coils producing the toroidal field.
These particles must be neutral in order to be able to over come the magnetic field without being deflected.
A known process then comprises firstly producing ions, then accelerating the latter in an electrostatic field, followed by the neutralization of the accelerated ions by charge exchange in a gas or vapour-filled cell. The thus formed fast neutral particles are able to pass through the magnetic barrier of the machine. On traversing the plasma they are ionized and rapidly degrade their energy by heating the plasma. Bearing in mind the effective interaction cross-sections between the beam of neutral particles and the plasma of a reactor previously defined by its density and dimensions, the energy per nucleon necessary for a 2 meter penetration is approximately 100 to 150 keV for hydrogen or 200 to 300 keV for the neutral atom of accelerated deuterium (D.sub.1).
The hitherto used processes comprise accelerating a light positive ion beam (H.sup.+ or D.sup.+) from a cooled grid ion source. The transparency of the grids is approximately 40 to 50% of the total surface of the source. Such sources produce beams, whose current density is approximately 2000 A/m.sup.2 and with a limited angular divergence (approximately 0.7.degree.).
The ions are then partly neutralized in a gaseous target produced in a tube of approximate length 2 m, whose cross-section is that of the beam. Under molecular conditions, a non-ionized gas from the ion source flows along this tube. This device constitutes a "Maxwellian neutralizer". The thickness of the target is characterized by the mean product of the density n by the length of the neutralizer l, said product being designated l. It is necessary to have approximately 10.sup.20 particles per m.sup.2 to obtain equilibrium neutralization between the neutral particles and the ions produced.
Such a gaseous target provides no supplementary divergence to the beam of ions and neutral particles. It also supplies no supplementary particles to the beams. However, it does form a gas charge, which it is necessary to pump by appropriate systems.
The neutralization efficiency of these systems is limited towards the high energy levels (200 to 300 keV), due to the relative decrease in the effective charge exchange cross-section when the energy increases. For D.sub.1.sup.+ ions of 300 keV, this efficiency is equal to r.sub.N =s.sub.10 /s.sub.10 +s.sub.01 or approximately 6%, in which s.sub.10 represents the effective capture cross-section of an electron by ion and s.sub.01 represents the effective ionization cross-section of a fast neutral particle. At the present state of the art and at these energy levels, such phenomena do not make it very efficient to use positive ions.
This is one of the reasons why at present another method is being envisaged and consists of using negative ions (H.sup.- or D.sup.-), which in principle it is possible to accelerate to 300 keV or more. The theoretical neutralization efficiency would then be approximately 60%.
However, on the one hand the current density of these sources is much lower and is approximately 200 A/m.sup.2 instead of 2000 A/m.sup.2 obtained with positive ions and on the other hand the acceleration and neutralization cause serious problems, due to the presence of very high electron currents accompanying the negative ions. Moreover, the neutralization of the H.sup.- ions is accompanied by a production of positive ions of the same energy as the H.sup.- and H.sup.o. Bearing in mind the low relative emission of the negative ions, the size of the negative ion sources is much greater than that of the sources supplying positive ions. Thus, neither of the two methods is really satisfactory.